Use __builtin_unreachable(). [skip ci]

Same as in libpcap.

diag-control.h

@@ -167,4 +167,20 @@
 #define DIAG_ON_C11_EXTENSIONS
 #endif
 
+/*
+ * GCC needs this on AIX for longjmp().
+ */
+#if ND_IS_AT_LEAST_GNUC_VERSION(5,1)
+  /*
+   * Beware that the effect of this builtin is more than just squelching the
+   * warning! GCC trusts it enough for the process to segfault if the control
+   * flow reaches the builtin (an infinite empty loop in the same context would
+   * squelch the warning and ruin the process too, albeit in a different way).
+   * So please remember to use this very carefully.
+   */
+  #define ND_UNREACHABLE __builtin_unreachable();
+#else
+  #define ND_UNREACHABLE
+#endif
+
 #endif /* _diag_control_h */

extract.h

@@ -37,6 +37,7 @@
  */
 #include "funcattrs.h"
 #include "netdissect.h"
+#include "diag-control.h"
 
 /*
  * If we have versions of GCC or Clang that support an __attribute__
@@ -585,7 +586,7 @@ nd_trunc_longjmp(netdissect_options *ndo)
 	 * XL C that the function is noreturn, but GCC remains unaware of that and
 	 * yields a "'noreturn' function does return" warning.
 	 */
-	while (1);
+	ND_UNREACHABLE
 #endif /* _AIX */
 }